1. Field of the Invention
The present invention specifically relates to an inkjet printhead having printing elements used for generating energy for discharging ink and driving circuits used for driving the printing elements formed on the same element substrate, as well as to a head cartridge and a printing apparatus utilizing the printhead.
2. Description of the Related Art
Generally speaking, the printing elements of a printhead installed in an inkjet printing apparatus, i.e. electrothermal transducers (heaters), as well as their driving circuits, are formed on the same substrate using semiconductor processing techniques as described, for example, in U.S. Pat. No. 6,290,334.
In recent inkjet printing apparatuses, there has been a tendency to provide a printhead with multiple nozzle arrays corresponding to inks of multiple colors for the purpose of improving, for example, the level of print quality and the speed of printing.
As an example of implementing such multiple nozzle arrays corresponding to inks of multiple colors, configurations exist, in which an element substrate used for discharging black ink for the purpose of printing text, etc., and an element substrate used for discharging color inks for the purpose of printing photographs, diagrams, and tables, etc., are installed in a single printhead.
Moreover, as another example, configurations exist, in which multiple element substrates are installed in order to implement printing in inks of multiple colors, such as 6 colors or 8 colors, for the purpose of improving the tonal characteristics and expanding the color space of the printed images.
The installation of multiple element substrates for the purpose of improving the level of print quality and the speed of printing is proposed as a characteristic feature of these recent printheads.
Element substrates with integrally formed heaters and driving circuits have various layouts. As an example, FIG. 4 shows an element substrate, in which pads of power supply terminals connected to the outside and signal terminals are disposed along the short sides of the element substrate and, from there, are connected to internal circuits through aluminum (Al) wiring.
Logic signals, such as supply voltage, image data, etc., are input from the pads 410 disposed along the short sides of the element substrate 400. As necessary, the input signals etc. are input to shift registers 403 through input circuits 402. The shift registers 403 convert the input serial data into parallel data. Image data, as part of the data converted into parallel data, is output to printing element (heater) selection circuit arrays 405, while the other portion of the data is output to decoders 404 as time-division data. In the decoders 404, time-division signals defining the timing used for driving the heaters are output to the printing element selection circuit arrays 405 including AND circuits and the like. The printing element selection circuit arrays 405 select the heaters to be turned on from among the heaters contained in heater arrays 407. The driver transistors of driver transistor arrays 406 corresponding to the selected heaters drive the respective heaters by supplying them with electric current, thereby resulting in the generation of a bubble and discharge of the ink.
Moreover, function circuits 105 are circuits having processing functionality used to provide more efficient drive control for the heaters. Examples of the function circuits 105 include circuits generating or converting the driver driving voltage used to drive the driver transistors, circuits generating a voltage used as a reference, circuits controlling a drive current fed to the heaters, circuits externally outputting information on the element substrate 400, such as temperature etc., memory units used to hold and store information on the element substrate 400, circuits used for processing and modifying various data, etc. Thus, the term “function circuits” refers not only to circuits used to transfer data and signals to various element substrates, but also to circuits performing processing functions, such as voltage generation and conversion, data processing and modification, data storage and output, etc.
It should be noted that while FIG. 4 shows an embodiment, in which the element substrate 400 has an ink supply port 104, there are abundantly available embodiments, in which a plurality of ink supply ports 104 are disposed on a single element substrate, permitting discharge of inks of multiple colors. Furthermore, printheads have been widely adopted that combine an element substrate used for discharging black ink (black ink element substrate), which has one ink supply port 104, and an element substrate used for discharging color inks (color ink element substrate), which has multiple ink supply ports 104.
Typically, in an element substrate 400 provided with multiple ink supply ports 104, there are disposed multiple printing element selection circuits and circuits used for drive control corresponding to the number of the ink supply ports 104. In addition, in such a case, the configuration of the circuits is implemented in an efficient manner suitable e.g. for sharing the circuits that can be shared within the element substrate 400.
In addition, there are printheads, in which NMOS transistor-based power transistors are utilized as driver transistors used to supply electric current to the heaters. A printhead configuration, in which the supply voltage (e.g. 3.3V or 5V, etc.) of a logic circuit is boosted and applied to the gates of NMOS transistors in order to improve the drivability of the NMOS transistors, has been disclosed in U.S. Pat. No. 6,302,504.
FIG. 6 is a diagram illustrating a conventional example showing an equivalent circuit of a single segment comprising one heater, one driver transistor, etc. It should be noted that while the converted voltage generation unit, as shown in FIG. 7, is provided so as to be common to multiple segments, the purpose of the description herein is to show connections, and therefore it is not provided for each single heater.
A heater-driving signal is output from a heater selection circuit 120 at a voltage equal to the supply voltage VDD of the logic circuit via a shift register (S/R), a decoder, etc., which are not shown in the figure. In a level converter 121, the heater-driving signal is boosted to a driver driving voltage VHTM, which is higher than the supply voltage VDD of the logic circuit, after which the signal is input to the gate of a driver transistor 123 through a driver driving buffer 122. This is done in order to reduce the effective resistance in the driver transistor 123 during the operation of the heater 124 by applying a voltage that is higher than the supply voltage of the logic circuit to the gate of the driver transistor 123. It should be noted that the driver driving voltage VHTM is generated by a converted voltage generation unit 125 located within the element substrate from a voltage VHT having the same potential as the heater supply voltage VH input from the printer main body.
FIG. 7 is a diagram illustrating a relationship between a converted voltage generation unit and driving circuits used for multiple segments provided on an element substrate in a printhead.
As shown in FIG. 7, each of the M segments is provided with a level converter, a driver driving buffer, and a driver transistor 123, with signals from the above-described heater selection circuit input to each of the segments. Moreover, the element substrate is provided with at least one converted voltage generation unit 125 that inputs, to each of the segments, the driver driving voltage VHTM generated from the voltage VHT having the same potential as the heater supply voltage VH.
FIG. 8 shows a circuit diagram of a converted voltage generation circuit.
The converted voltage generation circuit of FIG. 8 (converted voltage generation unit 125) is configured such that the driver driving voltage VHTM is output by a source follower circuit that applies the desired voltage, which is set depending on the resistance ratio of resistors 130 and 131, to the gate of a MOS transistor 132. The voltage value of the driver driving voltage VHTM is determined by the ratio of the resistance values of the resistors 130,131 and voltage VHT, which has the same potential as the heater supply voltage VH.
In general, inkjet printheads have been regularly modified to increase the number of nozzles and their density in order to raise the speed of printing and enhance the quality level of the printed images. However, for example, in thermal inkjet printers, in which the discharge of ink is carried out by generating heat with the help of heaters, the temperature of the element substrate rises because the heaters are caused to generate energy required for discharging the ink. Since the rise in the temperature of the element substrate affects the generation of a bubble and discharge characteristics of the ink, it becomes necessary to adjust the energy applied to the heaters while monitoring the temperature of the element substrate. However, in some cases, when the temperature of the element substrate becomes higher than the temperature, at which the energy imparted to the ink can be regulated, the printing operation may have to be temporarily suspended to allow the element substrate to naturally cool.
The effects of the rise in element substrate temperature tend to be particularly pronounced in element substrates used for color inks, where the droplets of the discharged inks have to be made smaller in order to implement a high level of quality during photographic image printing.
In addition, the feature whereby ink discharge is carried out by heating with heaters requires an electric current of several tens to several hundred mA to flow through each heater. For this reason, as the number of nozzles that simultaneously discharge ink increases, concern arises that the current noise caused by the current flowing to these heaters may affect the operation of the function circuits provided on the element substrate.
Moreover, there are cases, for instance, such as when photographic images are printed, in which the black ink element substrate is practically inactive and the operation of the color ink element substrate takes center stage. On the other hand, there are cases, such as when text images are printed, in which the color ink element substrate is practically inactive and the operation of the black ink element substrate takes center stage. Thus, there are cases, in which the used element substrate is just one of the substrates. On the other hand, due to the fact that printheads having conventional separate black and color element substrates had function circuits provided for each one of the element substrates in order to impart various functions to the element substrates, circuit operation also tended to concentrate on a single element substrate. For this reason, the effects of the current noise and the rise in element substrate temperature were sometimes manifested in a biased manner in only one of the element substrates.